Answer
$t_{\frac{α}{2}}=t_{0.05}=1.706$
Work Step by Step
$n=27$, so:
$d.f.=n-1=26$
$level~of~confidence=(1-α).100$%
$90$% $=(1-α).100$%
$0.9=1-α$
$α=0.1$
$t_{\frac{α}{2}}=t_{0.05}=1.706$
(According to Table VI, for d.f. = 26 and area in right tail = 0.05)
